Noncontact rotary position sensing apparatus are known for determining the rotary position of a shaft or the like. Some sensing apparatus employ a Hall effect device in combination with a permanent magnet. The Hall effect device is typically mounted to a stationary member adjacent the shaft. A permanent magnet is mounted to the shaft. The Hall effect device generates a voltage indicative of the relative rotation between the stationary member and the shaft.
One known apparatus utilizes an annular ring magnet having a plurality of circumferentially oriented poles. The ring magnet is coaxially attached to a rotary shaft and a Hall effect device is attached to a stationary member adjacent the ring magnet. In particular, the ring magnet includes a plurality of magnets connected in series to form a ring with the north and south poles of the magnets alternately arranged. As the shaft and the attached ring magnet rotate, the Hall effect device generates a sinusoidal electric signal indicative of the magnetic induction or the magnetic flux density produced by the magnets at the Hall effect device. The polarity of the magnetic flux density and the generated electric signal changes as each pole passes the Hall effect device. The number of signal cycles per revolution of the ring is a function of the number of poles that make up the ring. A counter counts the number of waveform cycles of the signal generated by the Hall effect device. The count is then used to determine the rotary position of the shaft. The accuracy of such a system is dependent upon the number of poles that make up the ring.
Noncontact rotary position sensing apparatus are particularly useful in an ink key control system for metering a fluid fountain in a printing press. To produce high quality printed matter from a printing press, it is essential that ink be consistently delivered in a controlled manner for deposition upon the paper or other stock in the printing operation. Control of the ink film profile across an ink roll requires an accurate means for metering the fluid film.
One ink key adjustment system for controlling the ink film profile on an ink roll is disclosed in co-pending U.S. patent application Ser. No. 629,616 to Kubert et al. The ink key metering apparatus of Kubert et al., includes a plurality of ink keys spaced along a flexible fountain blade. Each ink key has a rotatable shaft that meters a portion of the fountain blade with respect to a fountain roll upon rotation of the shaft. Each ink key shaft has a multipolar ring magnet coaxially mounted thereto. A Hall effect device is mounted to a traveling carriage that is selectively positionable adjacent a ring magnet of a selected ink key. A sinusoidal electrical signal is generated by the Hall effect device as a selected ink key is driven through a drive arrangement and the ring magnet rotates relative to the Hall effect device. The rotational position of the ink key shaft can be determined from the generated signal which is indicative of the relative position between the fountain blade and the fountain roll. The accuracy of such a system is limited by the number of poles that make up the ring magnet.